Smart batteries in series...is it even possible?

[kholmz]

Is it even possible to connect LiFePo4 batteries in series if they have BMSs? I can see two issues. 1) When charging, will the battery that charges first go into high impedance and prevent the other batteries from charging? 2) When charging, will the battery that charges first go into high impedance and therefore the full voltage of the charger will be build across that battery and cause it to go into the overvoltage protection mode requiring a reset?

 

[sunshine_eggo]

Yes. This is done all the time. Better to get a bigger BMS and go with a 24V or 48v battery as applicable.

1) Yes. That's why it's critical that the 12V be fully charged and top balanced to each other. If they are all at 100% charge at the same time, this will not happen.
2) This can cause a system voltage spike as the battery is removed from the charge circuit, and the charger can't cut to 0 amps quickly enough.

Something that you may not understand is that BMS are safety equipment, not functional equipment, i.e., you should program the hardware to work within the limits of the BMS, so the BMS never gets used unless something goes wrong.

 

[cedarshoot]

Yes. This is done all the time. Better to get a bigger BMS and go with a 24V or 48v battery as applicable.

1) Yes. That's why it's critical that the 12V be fully charged and top balanced to each other. If they are all at 100% charge at the same time, this will not happen.
2) This can cause a system voltage spike as the battery is removed from the charge circuit, and the charger can't cut to 0 amps quickly enough.

Something that you may not understand is that BMS are safety equipment, not functional equipment, i.e., you should program the hardware to work within the limits of the BMS, so the BMS never gets used unless something goes wrong.

I’ve seen people build huge banks with the Valence batteries and wondered about this too….I have small system with two Valence 12v in parallel (bought right when Wills first Valence video dropped) and the bms are quick to protect at low voltage , but I’ve never sleuthed out which one hits first….the 12v Valence are waaaay cheaper now and it’s tempting to build a big system if not for this question.